Reversibility of the phosphate transfer between ATP and phosphoproteins catalysed by a cyclic nucleotide independent (G type) casein kinase.

Purified casein kinase G was found able to catalyse the synthesis of [gamma-32P]ATP in the presence of ADP, phosphocasein (previously 32P-labeled by the forward kinase reaction) and magnesium. Apparent Km values of approx. 0.5 mM for phosphocasein and 7.5 mM for ADP were calculated, these values indicating low affinities for the substrates as compared to those exhibited for casein and ATP in the forward reaction. The reverse casein kinase G activity appeared to prefer ADP and GDP as phosphate acceptors. Whereas the casein kinase G reverse reaction could be supported by casein, phosvitin and histone previously phosphorylated by the enzyme, the same proteins could not serve as a phosphate source when previously phosphorylated by the cAMP-dependent protein kinase. Forward and reverse casein kinase G reactions exhibited different optimal pH values (8.5 and 7.2, respectively) and a different sensitivity to Mg2+. Spermine, which activated the kinase activity, blocked the reverse reaction at millimolar concentrations. Although the biological significance of the casein kinase G reverse activity remains to be assessed in intact cell, the process may be useful as a tool in the characterization of phosphorylatable sites in phosphoproteins.